This invention relates to dynamoelectric machines, such as large turbine generators, whose end turns are supported in an assembly that includes threaded fasteners.
For general background, reference is made to Dailey et al. U.S. Pat. Nos. 4,379,243, Apr. 5, 1983 and 4,415,825, Nov. 15, 1983, and pending application Ser. No. 595,025, filed by Dailey et al. March 30, 1984 and assigned to the present assignee, whose descriptions are representative of the use of threaded fasteners in the end winding assemblies of dynamoelectric machines. As such designs have developed, they involve numerous fasteners including threaded studs and nuts for such purposes as anchoring top and bottom coils to a support cone, mounting a parallel ring structure to the end winding, and attaching radial bracing for the support cone.
The use of such fasteners presents two kinds of potential problems. One is that the nut may, after initial tightening, become loose as materials of the joined structure age and have been subjected to the forces encountered in machine operation. Without frequent retightening this may shorten the life of the components intended to be secured. Therefore, a desirable fastener should prevent loss of bolt load through loss of torque on the nut or reduce this loss to a sufficiently slow rate to allow retorquing of the hardware at reasonable service intervals.
Secondly, it is possible that a loose nut will not only fail to hold the assembly as desired, but that it will become a loose object in the machine that can lead to other types of wear and ultimate failure.
U.S. Pat. No. 4,379,243 (FIG. 8) shows a U-shaped retainer over the nut. Cords looped through holes in an adjacent component and over parts of the retainer hold it in place. This device keeps the nut from coming entirely off the stud but has minimal effectiveness against loosening of the nut.
In U.S. Pat. No. 4,415,825, it is disclosed that a nut on a stud end may have a pin inserted through a hole in it into one or more other elements of the joined structure. This secures the nut but further complicates the assembly through the need of forming and aligning the holes for the pin and ensuring the pin stays in place. Such a pin is not readily removed for retightening the nut. Also, that patent shows an example of a nut being retained on a stud through the use of a hardenable fabric such as a thermosetting tape that is wrapped around the nut and stud end after the nut is initially secured in place. This has not been found very successful in locking the nut but it does provide assurance against the nut becoming a loose missile in the machine.
A nut lock, which is not disclosed as applied in a dynamoelectric machine end winding assembly and would not be suitable for such purposes, is disclosed in Graffius U.S. Pat. No. 778,030, Dec. 20, 1904. The nut lock is a soft metal wire coil wound on the threads of a bolt against a nut. While such a device may slow the loosening of an adjacent nut, it is of limited value as the coil can be unscrew in a manner similar to that of a second nut.
The foregoing are representative of the varied attempts over a long period of time to provide a reliable, easy to use, locking device for nuts on threaded bolts or studs, particularly for nuts on fasteners in end winding assemblies of dynamoelectric machines.
Therefore, it is desirable to provide dynamoelectric machine end turn winding support assemblies with fasteners having locking devices that are easy to implement and which provide both assurance against loosening of the nut as well as the possibility of the nut becoming totally separated from the stud.
It is further desirable that the locking device employed on the fasteners be both easy to apply and to remove so service in the field can be performed to retighten the structure because materials have shrunk.
In accordance with the present invention, an end turn support assembly of a dynamoelectric machine is provided that includes fasteners holding the elements thereof together that have a stud with screw threads, a nut engaged on the stud screw threads, an outer portion of the stud extending beyond the nut, and a locking device on the outer portion of the stud for securing the nut against loosening in service, with the locking device comprising one or more layers of cord wrapped and tied on the stud outer portion screw threads and a cured coating of resinous material on the layers of cord. The stud outer portion preferably has a flat surface area, without threads, underlying the cord. The cord conforms to the uncylindrical configuration of that part of the stud to prevent the cord from turning on the stud like a nut. Also the stud outer portion preferaly has a layer of lubricant material on it underlying the cord to permit easy removal of the cord, and hence the nut, when desired simply by severing the cord and pulling it off. Washers, such as Belleville type spring washers, may also be used on the stud. All parts of the fastener assembly are preferably nonmagnetic and nonconducting.